The present invention relates generally to devices for measuring time intervals, and relates more particularly to a dual-speed ramp time interval meter including circuitry for automatic calibration.
Time interval meters for measuring time intervals in the sub-microsecond range have existed in the prior art. One such time interval meter is disclosed in U.S. Pat. No. 4,301,360 issued Nov. 17, 1981 to Bruce W. Blair and entitled "Time Interval Meter." The Blair time interval meter utilized a capacitor and two constant current sources to charge the capacitor to a predetermined reference voltage in two stages, a fast ramp period followed by a slow ramp period. During the fast ramp period, equal in duration to the time interval to be measured, the capacitor was rapidly charged by a first constant current source. After the end of the time interval, the first constant current source was switched off to end the fast ramp period and a second constant current source was switched on to begin the slow ramp period. During the slow ramp period, the capacitor continued to charge at a slower rate until the reference voltage was reached. The time duration of the slow ramp period was measured by counting pulses of a constant frequency clock and then multipling by the ratio of the two currents to obtain a measurement of the time interval. The Blair time interval meter thus utilized a dual-speed ramp technique to measure short time intervals.
The measurement accuracy of dual-speed ramp time interval meters is dependent upon the stability of the constant current sources. In calculating the measured time interval, the measured duration of the slow ramp period is multiplied by the ratio of the currents of the two constant current souces. This ratio is predetermined according to the nominal operation of the two current sources and is assumed to have a constant value. Any change in the actual current flow of either constant current source changes the actual ratio of the currents and, accordingly, the relationship between the durations of the fast and slow ramp periods. Since the ratio is assumed to be constant, such a change in the actual ratio will result in erroneous time interval measurements. Changes in the operation of the constant current sources can occur, for example, due to changes in temperature.
The measurement accuracy of dual-speed ramp time interval meters that charge a capacitor to a reference voltage is also dependent upon the stability of the capacitor and of the reference voltage. Even if the ratio of the currents is stable, variations in the capacitance of the capacitor or in the reference voltage will affect the relationship between the durations of the fast and slow ramp periods and, accordingly, the accuracy of the measured time interval.
It would be desirable, therefore, to provide means for accurately calibrating time interval meters to compensate for changes in the operation of constant current sources utilized therein. It is also desirable to provide a time interval meter having a measurement accuracy that is not dependent upon the stability of capacitors or reference voltages.